Orthogonal Frequency Division Multiplexing (OFDM) is a linear modulation scheme in which a set of orthogonal subcarriers are used to carry user data. OFDM can be efficiently implemented using Fourier transforms, such as the fast Fourier transform (FFT), and can be designed to be especially robust to multipath. Low complexity frequency domain equalization algorithms can be used to mitigate the impact of the channel. OFDM may provide a spectrally efficient scheme in which the subcarriers can overlap but remain orthogonal (in the absence of synchronization errors).
OFDMA (orthogonal frequency division multiple access) is an extension of OFDM in which multiple users share the same transmission band. OFDMA is a multi-user version of the OFDM modulation scheme. The multiple access feature is achieved in OFDMA by assigning subsets of the subcarriers to individual users, and allows the simultaneous low data rate transmission from several users over the same band. In conventional OFDMA, a different number of the available sub-carriers can be assigned to different users, in order to support differentiated quality of service (QoS).